Measurement of blood flow in the heart and vessels using the Doppler effect is well known. Whereas the amplitude of the reflected waves is employed to produce black and white images of the tissues, the frequency shift of the reflected waves may be used to measure the velocity of reflecting scatterers from tissue or blood. Color flow images are produced by superimposing a color image of the velocity of moving material, such as blood, over the black and white anatomical image. The measured velocity of flow at each pixel determines its color. The process by which by which black and white images are formed is conventionally referred to as B-mode imaging, while the process by which flow velocity is imaged using colors is conventionally referred to as color flow imaging.
During B-mode only imaging, ultrasound machines fire a set of B-mode acoustic vectors to obtain real-time images of anatomy with the human body. Likewise color flow imaging requires the firing of a set of color flow vectors. This requires the ultrasound machine to fire both B-mode and color flow vectors. The addition of the color flow vectors to the B-mode firings decreases the acoustic frame rate significantly. This decreases the update rate of the image data to an unacceptable level, which detracts from the real-time nature of the image and is a distraction to the user.
FIG. 3 shows an approach used in a conventional ultrasound machine to increase the apparent frame rate of the color flow data. In this approach, the apparent acoustic frame rate is increased by performing a 2:1 interpolation of the incoming acoustic data. The input to the interpolator 50 is the acquired color acoustic data, which is input at the acoustic frame rate. The interpolator generates additional color acoustic data sets by interpolating between each pair of acquired acoustic data sets. The interpolator output, which is at two times the acquired acoustic frame rate, is stored in a single acoustic frame buffer 52. The acoustic data is then read by a scan converter 54. A scan converter converts the acoustic image data from polar coordinate (R-.theta.) sector format or Cartesian coordinate linear array to appropriately scaled Cartesian coordinate display pixel data at the video rate. This scan-converted acoustic data is then output for display on a video monitor 56. The method effectively doubles the apparent frame rate of the color flow data.
The limitations of this prior art approach are two-fold. First, the 2x interpolation does not sufficiently improve the apparent acoustic frame rate in many cases. The second limitation lies in the use of a single acoustic frame buffer for storing and outputting the acoustic data. Because acoustic data is continuously written into the acoustic frame buffer, at any given time the acoustic frame buffer will contain data from more than one acoustic frame. The scan conversion process, which reads acoustic data out of the acoustic frame buffer, will therefore create images for display that contain data from two or more different acoustic frames. This results in an artifact in the image in which there is a distinct boundary between old and new data in the image. Because the acoustic frame rate and the video rate are asynchronous to each other, the artifact will precess through the image based on the ratio of the acoustic frame rate and the video rate. This "swirl" artifact is particularly noticeable when there is movement in the anatomy being imaged or if there is movement of the ultrasound transducer. The swirl artifact precesses through the image as new acoustic frames are acquired, scan converted and displayed.